The present application relates to the art of the LED lighting systems that produce visible light. It finds application in general purpose lighting and will be described with particular reference thereto. Those skilled in the art will appreciate the applicability of the present application to a variety of applications such as ornamental, special effects lighting, and other.
Typically, the LED lighting systems, which produce white or visible light incorporate blue LEDs, are coated with phosphor that converts some of the blue light radiation to a complimentary color, e.g. yellow-green emission. Combined blue, yellow and green emissions produce a white light.
Some white LED lighting systems unitize a blue LED chip coated with phosphors which are designed to convert some of the blue radiation into complementary colors. An example of a commonly used phosphor for this purpose is the yellow emitting yttrium gadolinium aluminum garnet phosphor activated with Ce3+. Other types of white LED lighting systems use an ultra violet emitting (UV) chip coated with a combination of phosphors producing white light.
To produce a white light of a correct color, a uniform coating of the phosphor powder of precise thickness is required. For the blue-emitting chips, a uniform coating of the prespecified thickness is required to allow a fixed amount of blue radiation to be transmitted through the coating to produce the correct whiteness of the color. In addition, the uniform thickness of phosphor coating is required to avoid some regions of the coated chip emitting predominately blue radiation and other regions emitting predominantly yellow radiation, which produce a pattern of varying colors in the projected light.
Likewise, the uniform phosphor coating and precise thickness are required for the UV emitting chips of the white LED lighting systems. A thicker coating is needed to avoid a bleed-through of large amounts of the UV, while an extra thick coating causes excessive scattering of the UV and visible light back onto the LED and surrounding structures. Some of the radiation, which is scattered or reflected back onto the LED, can be absorbed by the chip, reflector, submount, and/or lead structure resulting in a decrease of the light output.
One method to produce a phosphor coating is to use a phosphor slurry which includes phosphor in a liquid medium, an epoxy, or a silicone. In a slurry, phosphor particles are distributed randomly but settle quickly which results in different phosphor thicknesses over the geometry of the coating. Another method is to spray the chip with the slurry as in the screen printing. Yet another solution is to electrophoretically coat the chip or spray coat the lens.
However, because the LEDs and LED components are small and often irregular in shape, it is difficult to obtain a uniform distribution of coating. The coating naturally drains off corners and high points in the LED structure. Furthermore, it is difficult to produce a uniform thickness from one part of the LED to another.
The present application contemplates a new and improved apparatus that overcomes the above-reverenced problems and others.